1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly to an improved liquid crystal display device using chiral smectic C (SmC*) liquid crystals.
2. Description of the Related Art
Conventionally, a twisted nematic (TN) liquid crystal display device has been generally used as a display device utilizing liquid crystals. In the construction of such a device, a pair of electrode substrates each of which is formed with an orientation film thereon are disposed opposite to each other so as to render the orientation directions thereof crossing each other with a predetermined angle, whereby liquid crystals interposed between the electrode substrates are aligned twisted. In such a TN liquid crystal display device, however, its driving margin grows narrower as its multiplexing progresses, resulting in poor contrast. SBE (Supertwisted Birefrengence Effect) liquid crystal display element has developed as an improved type of the above display device. Although the contrast is improved, this improved display device still has disadvantages such as a limited viewing angle, a low response speed, and the like.
In 1980, Clark and Lagerwall proposed a display device using ferroelectric liquid crystals so as to solve the above problems involved in such conventional liquid crystal display devices using nematic liquid crystals [Appl. Phys. Lett., 36, 899 (1980)]. This liquid crystal display device is adapted to realize optical switching by changing the orientation direction of a liquid crystal molecule having optical anisotropy with a help of rotation caused when polarity of spontaneous polarization of a ferroelectric liquid crystal is to align with polarity of an electric field. One feature of this display device is bistability by virtue of which one optically stable state can be switched to the other or vice versa by applying a positive or negative electric field. Once an electric field is applied, display can be maintained even after removal of the electric field with the help of the bistability (memory effect). Another feature of the display device is that its switching speed is more than 1000 times as high as that of a TN liquid crystal display device. Utilizing this high switching speed characteristic and the above noted "memory effect" materializes high capacity display using matrix electrodes.
In manufacturing such a liquid crystal display device of nematic or ferroelectric type, liquid crystal molecules are required to be oriented in substantially parallel with the substrates and in one direction. Thus, in the conventional nematic liquid crystal display device, a polyimide film is typically used as an orientation film for the molecules which has been subjected to rubbing with cloth in one direction.
However, sufficient contrast cannot be obtained if the polyimide orientation film of the type used in the conventional TN liquid crystal display device is to be used in the display device using ferroelectric liquid crystals. For explaining this poor contrast, it is said that, in the display device proposed by Clark and Lagerwall, liquid crystal molecules having been considered to be oriented in one direction as shown in FIG. 4 are in fact not oriented in one direction, molecules near the upper substrate being oriented in different direction from those near the lower substrate as shown in FIG. 5, resulting in a twisted alignment of the molecules.
Orientation of the molecules is assumed to be mainly dependent of the nature of an orientation film contacting liquid crystals. Especially, such an orientation film as to attract strong or repel strong spontaneously polarized liquid crystal molecules tends to cause such a twisted alignment of the molecules. Therefore, to improve the liquid crystal display device in its contrast it is important to select an orientation film such as not to seriously affect the orientation of the spontaneously polarized liquid crystal molecules.